The University of Hong Kong's pathology lab is one of the few places on earth where you can stare a newly accused mass-murder suspect right in the face. Researchers there are using a powerful transmission electron microscope to examine a virus that was unknown to science just a month ago. This minuscule particle of protein-encrusted RNA is almost certainly the microbe that had, by last Saturday, infected more than 2,400 people in 19 countries  including up to 115 in the U.S.--and killed at least 89 since it began its rampage through the human population in China last fall. Projected onto a green phosphorescent screen by a beam of electrons and viewed through an optical device that resembles an upside-down periscope, the virus particles  this particular batch came from a relative of the Chinese doctor who became Hong Kong's first fatality on March 4--look chillingly like aliens in a sci-fi film. "There are the little buggers," says Dr. John Nicholls, a pathologist at the University of Hong Kong, leaning aside so a TIME reporter can take a look.

Magnified 100,000 times, the organisms are fuzzy little balls that fill the screen and look like the burrs that stick to your pants during a hike through the woods. You can just make out tiny hooks poking out of the spherical bodies  a telltale characteristic that helps classify the pathogen as a member of the coronavirus family. And while coronaviruses normally cause nothing more serious than a cold, the microbes on Nicholls' slide have evidently, for reasons researchers have yet to discover, mutated into a sometimes deadly infectious agent that has terrified the entire planet.

Since news of the first cases of SARS  short for severe acute respiratory syndrome  began emerging from mainland China a month ago, health authorities around the world have gone on high alert. For the first time in its 55-year history, the World Health Organization (WHO) recommended last week that travelers avoid nonessential trips to an entire region  China's Guangdong province and Hong Kong, right next door  for fear that they might contract and further spread the infectious agent.

But fear of the disease has already outpaced officials' warnings. Tens of thousands of Hong Kong residents venture outdoors only with their faces covered with surgical masks. Airlines have canceled flights in and out of China and Southeast Asia, and financial losses from reductions in tourism, retail spending and other business activity could reach billions of dollars. Schools were closed across affected regions, and last week authorities in Hong Kong forced 240 people from the hard-hit Amoy Gardens apartment complex into quarantine camps outside the city. In Canada, hospitals have closed to control the disease's spread, and health officials everywhere are looking hard at anyone who flies in from Asia and quarantining those with symptoms of infection  a fever of 100.4°F or higher, headaches and body aches, a dry cough and shortness of breath.

In the U.S., meanwhile, SARS is the only story other than the war in Iraq that has made it onto just about every news broadcast and front page. Even though nobody has died in the U.S. or even, with a single exception, got extremely sick, Americans are worried that this country could be the next major stop on SARS' international grand tour (see box), prompting President Bush to put SARS on the short list of quarantinable diseases last week.

All of this may seem like an overreaction to an illness that is not nearly as deadly as West Nile virus  SARS kills 3.7% of its victims, compared with West Nile's mortality rate of 6.7% last year  and evidently much less contagious than measles or even the flu. "It's the type of disease that seems to require a lot of direct close contact with somebody who's pretty sick," says Dr. Stephen Ostroff, deputy director of the National Center for Infectious Diseases at the Centers for Disease Control (CDC) in Atlanta.

But what makes SARS so frightening to scientists and the public is how much is still unknown about it, including where it came from, exactly how it spreads, how long its incubation period lasts (and thus how long a victim has been contagious when symptoms appear) and whether a vaccine will ever be available. Infectious-disease specialists are haunted by the great Spanish Influenza pandemic of 1918-19; it killed fewer than 3% of its victims but infected so many that at least 20 million people died in just 18 months  more than were killed in combat in World War I. And until health officials know for sure what they're dealing with, they tend to be overcautious. "When you confront new diseases and they begin to travel widely," says WHO spokesman Dick Thompson, "you have to do everything you can to try to stop the transmission."

If not for the secrecy of the Chinese government, health officials could have acted a lot earlier. It was back in November that a mysterious respiratory illness began spreading through the southern province of Guangdong. Officials hushed up the outbreak to prevent panic, and by February at least 305 Guangdong residents had developed SARS, according to Chinese officials. More cases are thought to have appeared in Beijing and other cities. By the time China finally turned in a two-page report to WHO a month later, the disease was already on the move.